8 & 9. Cable Selection Flashcards

1
Q

Explain the circuit design process

A
  1. Determine the design current (Ib).
  2. Select the rating for the protection device (In).
  3. Calculate the cable rating (Iz).
  4. Select a cable size from BS 7671 which will carry the current rating (It).
    - Make sure to pick the correct installation method.
  5. Calculate Voltage Drop (Vd)
  6. Check for Shock Risk constraints.
  7. Check for Thermal Constraints
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2
Q

What is the allowable volt drop?

A

3% for lighting circuits

5% for LV power circuits

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3
Q

When checking for thermal constraints, what is K?

A

70 Degrees: 115
90 Degrees: 143

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4
Q

What is the Maximum Ze for:

A TN-S Earthing System
A TN-C-S Earthing System

A

TN-S: 0.8 Ohms
TN-C-S: 0.35 Ohms

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